Activists shout slogans before the annual March for Life passes by the supreme court in January 2015.
Photograph: Alex Wong/Getty Images

The rate of abortion in the US reached a lower level in 2014 than in any other year since the procedure first became legal, a study has found, a decline that appears to be due to the widespread use of contraception producing a drop in unintended pregnancies.

Nineteen percent of pregnancies ended in abortion in 2014 – the lowest abortion rate since the supreme court handed down Roe v Wade in 1973, legalizing the procedure – and the number of abortions between 2011 and 2014 also fell, by 12%.

But the researchers found strong indications to link the decline in the abortion rate to the wider availability of highly effective contraception – which could be imperiled by efforts to repeal Obamacare by the incoming Republican administration.

The researchers made an estimate of the number of abortions by surveying local health department data and abortion clinics, which may be hampered by clinics that did not respond. Guttmacher is a thinktank that supports access to reproductive care, but its data is widely trusted by supporters and opponents of abortion rights alike.

The decline in the abortion rate was greatest in the midwest, south and north-east. Abortion is still a common procedure – in 2014, Jones and Jerman estimate, US women had 926,200 abortions – but there were nevertheless shifts in how abortions were performed. The number performed with medication, which is only effective early in a pregnancy, rose 7% to account for 31% of abortions outside a hospital setting.

There are competing theories to explain the decline in the abortion rate. The drop coincided with the enactment of the Affordable Care Act (ACA), which made more effective and expensive methods of contraception, such as IUDs, available to millions more women for no “copay” or prescription charge. But the decline also aligned with a historic spike in new, state-level abortion restrictions.

Some data – such as trends in contraception usage – that could help determine the reasons for the decline are not yet available for 2014. Still, the researchers predicted that the drop in the abortion rate had less to do with new restrictions than with changes in contraception usage and a reduction in unintended pregnancies.

One clue is that more than 60% of the decline in the abortion rate took place in states that had not enacted new hurdles to getting the procedure.

If the drop is due to contraception, it would have alarming implications for Republicans’ breakneck campaign to repeal the ACA. The law says that most health insurance plans must cover a broad range of contraceptive drugs and devices at no copay – the so-called contraception mandate. Public health advocates have credited this provision with an explosion in women’s access to more affordable and more effective birth control.

Between the fall of 2012 and spring 2014, a separate Guttmacher study found, the share of privately insured women who had no copay for contraception quadrupled. By 2015, the federal Department of Health and Human Services (HHS) found, 55.6 million US women had access to FDA-approved methods of contraception without a copay.

If Republicans were to repeal Obamacare, it is not clear that their replacement would contain a similar provision. Tom Price, Trump’s nominee to lead the HHS, has put forth several proposals for an Obamacare replacement that do not contain a contraception mandate.

Separately, the vice-president-elect, Mike Pence, has proposed issuing a rule, through the HHS, that would allow business owners to refuse to cover contraception if doing so violates religious beliefs.

“Their agenda … could stop or reverse progress in empowering women to meet their childbearing goals, including by avoiding unintended pregnancy,” Joerg Dreweke, of Guttmacher, wrote in a policy brief accompanying the new study.

“There is strong evidence from recent abortion declines that supporting women’s decision-making across the spectrum of reproductive healthcare is very much compatible with reducing abortion incidence.”

The Guttmacher researchers found less evidence to link the decline in abortions or to new abortion restrictions. About 38% of the decline in the number of abortions was observed in 22 states that had enacted new restrictions significant enough to potentially impact women’s access to abortion.

Telling the story behind Roe v Wade: 'The play illuminates choice'

Read more

These included laws that imposed extra counseling for an abortion. But only eight of those 22 states had abortion declines that outranked the national average, and four states – Arkansas, Michigan, Mississippi and North Carolina – actually saw an increase in their abortion rates.

There was, however, one type of abortion restriction that seemed to cause a decline in the abortion rate. These were laws that placed medically unnecessary regulations on abortion clinics in order to shut them down.

On Tuesday, groups opposed to abortion linked the decline in the rate and number of abortions to laws that impose new restrictions. Clarke Forsythe, the acting president of the legal group Americans United for Life, said in a statement that those laws were a factor.

“Another factor in lowering the number of abortions is the power of beautiful pictures of life inside the womb, through ultrasound,” he said. “Such pictures are worth more than a thousand words when it comes to helping people understand whose lives are on the line.”

“Although Planned Parenthood and the abortion industry wants to say that this is a contraception story, the fact is, contraception has been around since the 1950s,” added Kristi Hamrick, a spokeswoman for AUL.

The decline in the abortion rate from 2011 to 2014 continues a long downward trend. The US also saw its abortion rate drop between 2008 and 2011, driven, according to Guttmacher, by a steep decline in unintended pregnancies, probably explained by improvements in the use of contraception. As the abortion rate fell, the birth rate did not rise commensurately.

In particular, the rise of the use of highly effective, long-acting, reversible contraception, such as IUDs, might account for the drop in abortions.